This invention relates generally to power transmission line protection systems, and more specifically concerns a line differential protection system for power transmission lines.
Modern power systems typically require high speed fault clearing to preserve the transient (short term) stability of the system and to provide better power quality by reduction in reduced voltage (voltage sag) duration. The most widely used fault protection systems satisfying such requirements for transmission lines, i.e. those power lines with nominal voltages of 115 KV and greater, are directional protection systems using directional comparison techniques. While the directional comparison approach has some advantages, including low channel (communication) requirements between relays positioned at the local and remote ends of the power line, along with inherent redundancy, it does require voltage values obtained from the power signal on the power line. Such systems experience problems (often severe problems) because of voltage errors or missing voltages caused by small voltage factors, including blown fuses in the system, problems with windings in the system voltage transformer (VT) devices and transient responses in the system capacitive coupled voltage transformers.
One alternative to directional comparison systems using voltage values is a current differential system, which uses only the electrical current value information from the power line. Current differential systems, also known as line differential systems, do not require voltage measuring devices, as they do not use voltage values in their fault determinations. Line differential systems are less sensitive to power swings and sudden load changes in the system and are generally less sensitive to or even immune from certain conditions on the line, including zero sequence mutual coupling effects and/or current reversals, among others. However, along with the advantages are several significant disadvantages, including reliance on high communication channel performance, which is required between the local and remote protective relays on the line. In addition, conventional line differential systems using phase current quantities are limited in their ground fault resistance coverage and also are a compromise to an extent in security under current transformer (CT) saturation conditions.
The present invention is a new line differential protection system which, while still dependent upon a communication channel, includes significant improvements relative to other system considerations, including high fault resistance coverage, improved operating characteristics and sensitivity, while at the same time maintaining power system security.
Accordingly, the present invention is a system for current differential protection for a power transmission line comprising: means for determining the phase current values at local and remote ends of a transmission line; means for calculating the magnitude value of the ratio of the remote phase current values and the local phase current values and for calculating the angle difference value between the remote current and local current values; comparison elements for comparing the ratio magnitude value and the angle value against preselected values which establish a phase region in a current ratio plane for restraining tripping of a circuit breaker for the transmission line, wherein when the magnitude value and angle value are not within the established phase region, an output signal is developed which is a tripping signal for the circuit breaker and wherein when the magnitude value or the angle value are within the established region, no trip signal is produced; means for determining a selected one of (1) negative sequence current values and (2) zero sequence current values at local and remote ends of the transmission lines; means for calculating the magnitude value of the ratio of the remote selected sequence current values and the local selected sequence current values; means for calculating the angle difference value between the local and remote selected sequence current values; and comparison elements for comparing the ratio magnitude value and the angle value against preselected values which establish a selected sequence restraint region in the current ratio plane, wherein when the ratio magnitude value and angle value are outside of the sequence region, an output signal is developed which is a tripping signal for the circuit breaker and wherein when the ratio magnitude value or the angle value are within the sequence region no trip signal is produced.
In a related aspect of the invention, the positive sequence current values are substituted for the phase current values and used in combination with the negative sequence current values with a selected one of the negative sequence current values and zero sequence current values.